Table of Contents

EncDotNet.S100.Datasets.NetCdf

EncDotNet.S100.Datasets.NetCdf reads NetCDF-CF wind and wave grids, such as Copernicus Marine wave forecasts and NOAA ERDDAP griddap downloads, into the shared weather grid model of EncDotNet.S100.Datasets.Weather. It's managed code with no native dependencies: NetCDF-3 is read by a small reader written from the Unidata classic format specification, and NetCDF-4 is read through PureHDF.

Not for navigation. Weather model data isn't official S-100 or ENC data. Hosts label it so wherever it appears.

The design is in Wind and wave data, §3.2 (detection), §3.4 (the grid model) and §7.1 (this reader). To draw, sample and pick a file, use NetCdfDatasetProcessor in EncDotNet.S100.Datasets.Pipelines, which the default product registry picks up as NetCDF.

Install

dotnet add package EncDotNet.S100.Datasets.NetCdf

Read a file

using EncDotNet.S100.Datasets.NetCdf;
using EncDotNet.S100.Datasets.Weather;

using var dataset = NetCdfWeatherDataset.Open("cmems_mod_glo_wav_anfc.nc");

Console.WriteLine($"{dataset.Format}: {dataset.GridSet.Origin.Model}, run {dataset.ReferenceTime:u}");
foreach (var parameter in dataset.GridSet.Parameters)
    Console.WriteLine($"{parameter.Id} ({parameter.SourceName}), {parameter.Steps.Count} steps");

// Steps are read from the file only when decoded, normalised to the
// south-west-origin grid, with missing values as NaN.
var height = dataset.GridSet.FindParameter(WeatherParameters.WaveHeight)!;
float[] values = dataset.GridSet.ReadStep(height, stepIndex: 0);

NetCdfWeatherDataset keeps the file open, because each time step is read when it's first decoded; dispose it to close the file. NetCdfFile is the format layer underneath, if you need the raw variables and attributes.

Formats

Format Signature How it's read
NetCDF-3 classic CDF\x01 Managed reader: header, fixed-size and record variables, 32-bit offsets
NetCDF-3 64-bit offset CDF\x02 The same, with 64-bit offsets
NetCDF-4 HDF5 signature PureHDF: dimension scales, DIMENSION_LIST, hyperslab reads
  • Record variables. A numrecs of 0xFFFFFFFF (a file still being written) is worked out from the file length.
  • Compression. NetCDF-4 deflate and shuffle are supported (PureHDF also reads Fletcher-32, N-bit and scale-offset). Any other HDF5 filter, such as Zstandard or Blosc, fails with a NetCdfFormatException that names it.
  • Not supported. CDF-5 (CDF\x05), NetCDF-4 groups (only the root group is read), and compound, enum, opaque and variable-length types. Each fails with a message saying so.

CF rules

  • Coordinates. Latitude, longitude and time are found by standard_name (latitude, longitude, time), by axis (Y, X, T) or by units (degrees_north, degrees_east, "<unit> since …"). Latitude and longitude must be 1-D and evenly spaced; rows running south and 0 to 360 longitudes are normalised by the weather model.
  • Time. Units of seconds, minutes, hours or days since a reference time, in the standard, gregorian or proleptic_gregorian calendar (CfTimeUnits). Months, years and other calendars fail.
  • Variables. A data variable on the latitude × longitude grid is matched to the weather parameter table by its standard_name and converted to the table's units (feet, knots, km/h and hPa are converted, and degrees Celsius and Fahrenheit to kelvin; anything else makes it an unknown parameter). Extra dimensions of length 1 (a single height or depth) are allowed; a 10 m height is the wind level. A variable with several levels is listed in IgnoredVariables and left out.
  • Values. _FillValue, missing_value, valid_min, valid_max and valid_range are applied to the stored values (and the NetCDF default fill value when there's no _FillValue), then scale_factor and add_offset.
  • Model and run. The model is the first of the global source, institution, creator_name and title attributes that's at most 80 characters long, so a source that lists input files or sensors (the HF radar currents name every radial file) is passed over. The run time is a forecast_reference_time variable or global attribute; without one, ReferenceTime is null and the grid set's run time is the first valid time.

Failures say what to do or what the file holds:

Exception When
NetCdfFormatException Not NetCDF, CDF-5, a damaged or truncated file, groups only, an unsupported filter
CfConventionsException No latitude, longitude or time coordinate; unsupported time units or calendar; no variable the parameter table knows. The message lists the variables found, with their standard names
UnsupportedWeatherGridException 2-D latitude and longitude (a curvilinear grid), a grid_mapping variable other than latitude_longitude (named in the message), uneven spacing, or a regional grid across the antimeridian. A grid_mapping that names a variable the file doesn't have is ignored: ERDDAP subsets leave it out, and the 1-D latitude and longitude already place the grid

Dependencies

  • EncDotNet.S100.Datasets.Weather: the grid model and parameter table.
  • PureHDF: the HDF5 reader under NetCDF-4.

See also

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